Viruses Coach Blocker Schley County Middle School Ellaville, Georgia

Characteristics of Viruses Viruses are pathogens – any agent that can cause diseasepathogens Among the smallest biological particles that are capable of causing diseases in living organisms Constructed of compounds associated with cells

Are viruses alive?

Not Considered Living A virus is not a bacteria, fungus, protist, plant or animal. They can not carry out cellular functions. A virus can not replicate without infecting cells and then using the organelles and enzymes of the host cell.

A Virus Has Two Essential Features A Nucleic Acid DNA or RNA But not both A Capsid – a protein coat surrounding the nucleic acid.

VIRUS STRUCTURE

Some may have a membrane like structure outside the capsid called an envelop Examples: Influenza Chickenpox Herpes-simplex HIV

Influenza virus, surrounded by an envelope with spikes

Adenovirus, a naked virus, with a polyhedral capsid and a fiber at each corner

Types and shapes

Viral Shape The shape of the virus is determined by either its capsid or its nucleic acid Two examples of shape Helix is a spiral shape ( like DNA) – examples are rabies, measles and tobacco mosaic virus Polyhedron has 20 triangular faces – examples are herpes simplex, chicken pox and polio

Helical Rod like with capsid proteins winding around the core in a spiral Tobacco Mosaic Virus

Polyhedral Has many sides Most polyhedral capsids have 20 sides and 12 corners

Polyhedral capsid attached to a helical tail.

Virion Structure

Flu virus

T4 Bacteriophage

HIV

There are Two Types of Viruses DNA Replicated in one of two ways Directly produce RNA that make new viral proteins Join with the host cell’s DNA to produce new viral proteins

RNA Viral RNA is released into the host cell’s cytoplasm and uses the ribosomes to produce new viral proteins Some are known as retroviruses containing an enzyme called reverse transcriptase. These use the RNA as a template to make DNA. This DNA is integrated into the host cell’s DNA.

Viral Infection

Viruses and Living Cells Viruses must infect a living cell in order to grow and reproduce They also take advantage of the host’s respiration, nutrition and all the other functions that occur in living things Therefore, viruses are considered to be parasites

How do viruses replicate? 2 methods of replication: 1. Lytic Cycle – the virus enters the cell, replicates itself hundreds of times, and then bursts out of the cell, destroying it. (Active cycle) 2. Lysogenic Cycle – the virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA. An environmental change may cause the virus to enter the Lytic Cycle. (latent cycle)

In the lytic cycle, the virus reproduces itself using the host cell's chemical machinery. (Active Virus)

In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions. (Latent virus)

Vaccines 1.Improves immunity to a particular disease 2.Often made from weakened or killed forms of the microbe or virus 3.Stimulates the body's immune system to produce antibodies to recognize the agent as foreign, destroy it, and "remember" itantibodies